In a trading update, the Maidenhead-based company said that it had won several major public sector orders since the beginning of the year.
A spokesman said: "Public sector work accounts for about 80 per cent of the company's revenue. The group continues to benefit from its strategic focus on targeted blue chip customers in chosen sectors."
The contracts include: £397m contract with the Greater Manchester Waste Disposal Authority, £400m contract with Severn Trent Water, and £200m joint venture road maintenance contract for the Highways Agency.
This week, Costain reached financial close on a school PFI contract in Lewisham, south London. The company has over £900m of work secured for 2009. It has also reached preferred bidder status on further contracts worth £800m.